Y'know - the only issue I'm concerned with when mixing different light strengths is that you lose the proportionality of your modeling lamps betweeen lights. This will ramble a bit but I'll try to explain my thinking. For example: If both studio lights are set to 100% of their power - each modeling lamp shines at 100% as well. Assuming your lights have the same modeling lamp - each of them projects the same light intensity onto your subject, but the power of each flash head is different. So you won't visually see the true comparisan between your 500 main light and 250 fill/shadow just by looking at your modeling lamps. Does that make sense?
Stated another way: If you set your 250 light at full power (100%) to the left of your subject, and the 500 light at half-power (50%) from the right of your subject ... the proportional modeling lamps would appear to give you more lightfrom the left side of your model's face, while the right side would appear a bit darker as filled shadow. Yet the resulting photos would have 250w/s of flash on each side - causing unexpected results from what you saw in your modeling lamp setup.
I know it's easy to merely calculate your outputs and lighting ratios between the main and fill using light settings, and just disregard the false impression your model lamps create in those instances ... but it's both easier and more predictable for me to stick with similar-strength lights and "see" the correct proportional results before pulling the trigger. Obviously well-experienced photographers wouldn't have any difficulty overcoming this, but I prefer to use either two Genesis 200's on my subject, or two Genesis 400's. Not a mix-n-match ... check your settings ... disregard your modeling lamps method. Just my $0.02
I haven't looked much at the Eli BXRi's yet. Maybe the modeling lamps are different wattages between the 250/500 so this would be a moot argument for you. But it's something to consider and check on in advance. I'd say go with both 500's, because for a few quid difference you have the extra power when needed but can easily stop one of them down or move 'em back to produce less light.